Motorized vehicles include a powertrain operable to propel the vehicle and power the onboard vehicle electronics. The powertrain, or drivetrain, generally includes an engine that powers the final drive system through a multi-speed power transmission. Many vehicles are powered by a reciprocating-piston type internal combustion engine (ICE) because of its ready availability, relatively-inexpensive cost, light weight, and relative efficiency. Such engines include four-stroke compression-ignited diesel engines and four-stroke spark-ignited gasoline engines.
Hybrid vehicles utilize alternative power sources to propel the vehicle, minimizing reliance on the engine for power, and increasing overall fuel economy. A hybrid electric vehicle (HEV), for example, incorporates both electric energy and chemical energy, and converts the same into mechanical power to propel the vehicle and power the vehicle systems. The HEV generally employs one or more electric machines that operate individually or in concert with an internal combustion engine to propel the vehicle. Since hybrid vehicles can derive their power from sources other than the engine, engines in hybrid vehicles may be turned off while the vehicle is stopped or is being propelled by the alternative power source(s).
Series hybrid architectures, sometimes referred to as Range-Extended Electric Vehicles (REEVs), are generally characterized by an internal combustion engine in driving communication with an electric generator. The electric generator provides power to one or more electric motors operable to rotate the final drive members. There may be no direct mechanical connection between the engine and the drive members in a series hybrid powertrain. The lack of a mechanical link between the engine and wheels allows the engine to run at a constant and efficient rate, even as vehicle speed changes. The electric generator may also operate to start the internal combustion engine. The system may also allow the electric motor(s) to recover energy by slowing the vehicle and storing it in the battery through regenerative braking.
Parallel hybrid architectures are generally characterized by an internal combustion engine and one or more electric motor/generator assemblies, all of which have a direct mechanical coupling to the transmission. Parallel hybrid designs utilize combined electric motor/generators, which provide traction and may replace both the conventional starter motor and alternator. The motor/generators are electrically connected to an energy storage device (ESD). The energy storage device may be a chemical battery. A control unit is employed for regulating the electrical power interchange between the energy storage device and motor/generators, as well as the electrical power interchange between the first and second motor/generators.
Electrically-variable transmissions (EVT) provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures, and also elements of traditional, non-hybrid transmissions. EVTs may be designed to operate in both fixed-gear (FG) modes and EVT modes. When operating in a fixed-gear mode, the rotational speed of the transmission output member is a fixed ratio of the rotational speed of the input member from the engine, depending upon the selected arrangement of the differential gearing subsets. EVTs are also configured for engine operation that is mechanically independent from the final drive, thereby enabling high-torque continuously-variable speed ratios, electrically dominated launches, regenerative braking, engine-off idling, and two-mode operation.
An EVT may combine the motor/generators with differential gearing to achieve continuously variable torque and speed ratios between the input and output. The EVT can utilize the differential gearing to send a fraction of its transmitted power through the electric motor/generator(s) and the remainder of its power through another, parallel path that is mechanical. One form of differential gearing used is the epicyclic planetary gear arrangement. However, it is possible to design a power split transmission without planetary gears, for example, as by using bevel gears or other differential gearing.
Hydraulically-actuated torque-transmitting mechanisms, such as clutches and brakes, are selectively engageable to selectively activate the gear elements for establishing different forward and reverse speed ratios and modes between the transmission input and output shafts. The term “clutch” is used hereinafter to refer generally to torque transmitting mechanisms, including, without limitation, devices commonly referred to as clutches and brakes. Shifting from one speed ratio or mode to another may be in response to vehicle conditions and operator (driver) demands. The “speed ratio” is generally defined as the transmission input speed divided by the transmission output speed. Thus, a low gear range has a high speed ratio, and a high gear range has a relatively lower speed ratio. Because EVTs are not limited to single-speed gear ratios, the different operating states may be referred to as ranges or modes.
The range or mode change may be controlled through a multi-clutch synchronization and release process. A first clutch associated with a currently-active range is carrying torque in an applied state, while a second clutch associated with a currently-inactive second range is carrying no torque in a released state. Shifting from a first range to a second range is accomplished by controlling the second, unapplied clutch to zero slip speed, and applying the second clutch (the oncoming clutch) thereby placing the EVT in a state with both clutches applied. The second range is then entered by the release of the first clutch (the offgoing clutch).